And, with more than 520 flights already grounded for Thursday, that put the week's cumulative cancellation tally at about 7,100 flights since Monday, according to FligthAware's count.

The hardest hit airport on Wednesday was Atlanta's Hartsfield-Jackson International, the world's busiest airport and the top hub for Delta Air Lines.

About 40% of the airport's flight schedule today - more than 530 departures and more than 547 arrivals -- had been canceled there as of 9:45 p.m. ET. That's partly fallout from Atlanta's problems on Tuesday, when more than 930 combined arrivals and departures were canceled. Many of Wednesday morning's cancellations resulted from planes or crew being out of position from Tuesday's disruption.

But it also came as the Atlanta metro area struggled to get going Wednesday after 2 to 3 inches of snow fell there on Tuesday. Many highways remained coated with ice as of mid-day and mass transit systems were running at reduced capacity. That was complicating efforts of airport workers attempting to get to the airport.

Delta and other airlines hoped to begin a recovery in Atlanta later today, working to get planes and crews back into their regular schedules. Still, especially with Wednesday's lingering problems, it could take airlines 24 to 72 hours, perhaps longer, to accommodate all of the fliers whose plans have been disrupted by this week's wintry weather.

That also was true nationwide, where delays and cancellations snarled thousands of flights at other key airports as well.

Chicago O'Hare and Houston Bush Intercontinental each had about 18% of their Wednesday flights grounded as of 9:45 p.m. ET, according to FlightAware. At Charlotte, the figure was about 13% All three airports are among the nation's biggest airline hubs.

Some smaller airports in the south, had 50% or more of their departures grounded in the fallout of the wintry weather. Among those: Norfolk, Va.; Charleston, S.C., Birmingham; Pensacola, Fla.; Savannah, Ga.; Lafayette, La.; Mobile, Ala.; and the Northwest Florida Regional Airport near Fort Walton Beach.

Many of the USA's busiest airports also were hit hard by delays and cancellations on Monday and Tuesday, including Houston Bush Intercontinental, Chicago O'Hare, Chicago Midway, Cleveland and others. Dozens of other, smaller airports from Nebraska to Texas to Florida and the Carolinas had substantial portions of their schedules scrubbed as a result of the storm.